FIELD OF THE INVENTION
This invention relates, in general, to the construction of sewing machines and, in particular, to a new and useful device for prepuncturing a workpiece which is being fed to a needle.
During insertion of a sewing machine needle into the workpiece, the needle pushes the individual fabric threads apart to make room for the knotting together of the upper and lower threads to be brought into the work. The fabric threads pushed aside try to return to their starting position as soon as the needle leaves the work, so that tensions form in the work during the stitch formation, which often lead to an undesirable wrinke or curl effect. To avoid this so-called displacement curling, it is known practice to use perforating tools which are arranged in spaced relation to and before the needle so as to create a prepuncturing in the work and to make room within the work for the knotting together of the upper and lower threads.
Austrian Pat. No. 275,291 indicates a heated point passing through the work from its underside, which causes the thermoplast portion of the work to melt in the zone of the subsequent insertion, to make room for the knotting of the upper and lower threads.
Known also from U.S. Pat. No. 3,241,508 is the provision, in the zone of the stitch forming point, of two electrodes, one being arranged above and the other below the work, to destroy the work material in the zone of the needle insertion by the controlled flashover of an electric spark.
Systems of the above mentioned kind are very expensive in their construction and especially for apparatus to effect their control. They require either a heating of the needle tip penetrating into the work dependent on the sewing speed or a control of the spark flashover dependent on the speed of work feed. In sewing machines operating at a speed of only about 3000 rpm, for example, only 20 milliseconds are available for a stitch forming process, and only few milliseconds remain for the prepuncturing to be carried out with the work standing still. Therefore, a control of the electric power for the spark flashover is extremely complicated even when the opening and closing of the circuit of the two electrodes occurs as a function of the rotary movement of the main shaft of the sewing machine. Moreover, due to the fact that in the known systems the creation of the holes is based on a thermal stress on the work material, they have the disadvantage that they can be used primarily or solely for work comprising a predominant amount of thermoplastic components.